The invention generally relates to mounting mechanisms for cutting inserts, and is particularly concerned with a mechanism for preventing the rotation of a round cutting insert mounted onto the body of a cutting tool.
The inserts used in tools such as milling cutters are mounted in complementarily-shaped pockets spaced around the periphery of the tool body. The inserts are typically secured within their respective pockets by clamping screws inserted through a hole provided in the center of the insert. During a cutting operation, such inserts often experience not only compressive and vibratory forces, but some amount of torque due to the angle between the cutting edges of the inserts and the workpiece. For cutting inserts of non-round shapes, such torque does not result in the rotation of the insert due to the interference-type fit between the angled sidewalls of such inserts and the complementarily-shaped walls of the pocket that receive them. By contrast, round inserts can rotate within their respective pockets since no such mechanical interference naturally arises between the cylindrical or frustro-conical sidewalls of round inserts and the circular walls of the pockets which receive them. The resulting rotation can loosen the clamping screw that secures the insert within its respective pocket. If the clamping screw should become sufficiently loosened, it can vibrate within the surrounding pocket severely enough to become chipped or cracked not only ruining the insert, but also jeopardizing the quality of the cut on the workpiece.
To prevent such unwanted rotation, several anti-rotation mechanisms have been developed in the prior art. In one such mechanism, a plurality of deep semicircular recesses are provided around the sidewall of the insert. A pin is provided adjacent to the sidewall of the insert-receiving pocket in the tool body that is complementary in shape to the recesses around the insert. In operation, the insert is installed in the pocket of the milling cutter or other tool with the pin of the pocket inserted into one of the semicircular recesses of the insert. During use, the insert is indexed to even out the wear on its cutting edges by periodically removing it from the pocket, partially rotating it, and reinstalling it so that the pin is received by a different recess. In another type of anti-rotation mechanism, a polygonal pattern of facets are provided around the circular sidewall of the insert. Such a mechanism is disclosed in U.S. Pat. No. 5,346,336. The insert-receiving pocket is provided with a complementarily-shaped pattern of polygonal walls for receiving the facets of the insert. The indexing of the insert is performed generally in the same manner as described in the first mechanism.
Unfortunately, a number of shortcomings associated with each design have been noticed. For example, the applicants have observed that, with respect to the first design, the edges of the full radius, semicircular recesses formed in the hard, carbide material of the insert can slowly cut the steel pin in the pocket as the result of vibrations between the pin and the edge of the recess, which is oriented substantially orthogonally with respect to the axis of the pin. Additionally, the provision of such deep recesses around the bottom of the insert body result in an undesirable weakening of the insert body as a result of the removal of the material necessary to form the recesses. The stop mechanism disclosed in the '336 patent likewise has shortcomings, albeit different ones. For example, the facets provided around the body of the insert are angled at a slightly steeper angle than the flat walls provided around the insert-receiving pockets to avoid contact between the walls of the pocket and the weaker, bottom portion of the insert. However, the limited contact provided by such a design can result in one-point contact between the edges of the insert facets and the flat walls of the pocket. Moreover, because the only contact between the insert and the pocket is along the upper portion of the insert, this relatively small area of the insert must absorb all the stresses generated between the insert and the pocket when the insert cuts a workpiece. Hence the limited and sometimes one-point contact between the insert and the pocket can result in localized chipping, cracking, or breakage of the insert over time.
To overcome these shortcomings, U.S. Pat. No. 6,238,133 describes an anti-rotation mechanism 100 having complimentary curved surfaces on both the cutting insert and the insert pocket. The operation of the anti-rotation mechanism 100 may best be understood with respect to FIGS. 8, 9, and 10. While the curvature between the stop surfaces 135 of the insert 103 and the anti-rotation surface 136 of the pocket 105 is substantially complementary, the two surfaces are deliberately spaced apart a distance, D, which is typically a few thousandths of an inch (see in particular FIG. 9). Such a small gap between the two surfaces 135, 136 allows the insert 103 to rotate an angle of between 1° and 10° after the insert 103 has been clamped into the pocket 105. Such a small latitude of rotational movement (i.e. a clocking) allows one of the oblique engagement portions 137a,b of the stop surfaces 135 to come into contact with one of the engagement portions 158a,b of the anti-rotation surface 136, as is illustrated in FIG. 10. The relatively gently sloping, large partial radius curves that define the stop surfaces 135 and anti-rotation surfaces 136 advantageously results in broad line or lenticular contact between the insert 103 and the nibs 154a,b. 
Referring now to FIGS. 11 and 12, the interaction between the stop surfaces 135 and the nibs 154a,b, can be explained by two circles; one circle with an inner diameter (I.D.) and one circle with an outer diameter (O.D). Now, move the I. D. until it touches the O.D. If the radius of the I.D. is reduced by a certain ratio, then it's location must be moved by the same ratio so that it will still touch the O.D. By doing this, it increases the remaining distance between the I.D. and the O.D. (dimension A). Unfortunately, this increased distance allows more rotational movement (i.e., clocking) of the cutting insert 103.